Production of sound



Dec. 4, 1934. E. w KELLOGG 1,983,377

PRODUCTI ON OF SOUND Filed Sept. 27, 1929 /2 IHVGTTIOT". L Edward \X/.Kellggg,

T0 STATIONARY b MEMBERS :J

His Attorney.

Patented Dec. 4, 1934 UNITED STATES PATENT OFFICE PRODUCTION OF SOUNDEdward W. Kellogg, Schenectady, N. Y., assignor to General ElectricCompany, a corporation of New York Application September 2'7, 1929,Serial No. 395,686

13 Claims. (Cl. 179-411) with the wattless volt-amperes required forcharging the electrodes to the. required varying potentials. This isespecially true at high frequency when the capacity reactance of theloud speaker becomes so low in comparison with the impedance of the.amplifier output circuit that the amplifier fails to produce adequatepotential variations between the loud speaker electrodes. The result isa loss in high frequency sound output compared with the low frequencyoutput. The response of a well designed electrostatic speaker of theusual type will be fairly uniform up to a certain frequency, and abovethis frequency the response, or sound output for a given voltage appliedto the amplifier input, falls rapid- 1y. The larger the mutual capacityof the loud speaker electrodes (for a given amplifier) the lower thefrequency at which this falling off in response begins to appear. It isthus incompatible with high quality sound reproduction to load anamplifier with an electrostatic speaker having more than a certaincapacity, and this factor sets the limit to the useful sound which canbe obtained from an amplifier of a given volt-ampere output rating.

In accordance with my invention these difiiculties are avoided bydividing the area into sections which have a comparatively smallelectrostatic capacity, and connecting these sectionsthrough inductancedevices. The arrangement may be regarded as'an artificial transmissionline or wave filter one end of which is connected to a high voltagesource. The other end'should be provided with damping resistanceasindicated in the aca companying drawing. By employing thisconstruction I simultaneously avoid two of the difil culties describedabove. Almuch larger total capacity may be employed without overloadingthe amplifier, and the impedance of the device, or in other words theload on the amplifier, is now the same at all frequencies within theessential audio range.

. The reason that this is possible is that it isnow no longernecessarytocharge all parts of the electrode area to fullpotentialsimultaneously, but the several sectionsreceive their charges inprogression, the charges being passed on from one section to the next.In fact by carrying the principle far enough, using a large number ofsections and low loss inductances, it is possible to build anelectrostatic speaker with which the amplifier has only to supply theenergy losses, which are in large measure due to sound radiation. Inpractice however it will frequently be desirable to stop short of thispoint, not employing a sufiicient number of sections to dissipate all ofthe energy supplied by the amplifier. The terminating resistancepreviously mentioned then serves to dissipate the residual energy andprevent wave reflections. Without such resistance, electrical resonanceis likely to occur, with resulting irregularitiesin response.

Since the device has the structure'of a-low pass filter there will be anupper frequency limit above which it will practically cease to radiate.This cut'oif frequency may be made as high as. 76 necessary by properlyproportioning the individual inductances and capacities. The cut off.frequency is that at which the inductive reactance of one coil is fourtimes the capacity reactance of one section of the speaker. The systemoperso ates with substantially unity power factor for the frequenciesbelow that at which it acts as a filter. The impedance of the device ispractically independent of frequency and is now very much higher thanpreviously, being now equal to twice the capacity reactance of a singleunit area, said capacity reactance being calculated for the highestfrequency in the essential range, namely the cut off frequency. With theproper arrangement of units no loss in quality is involved inthe employ-9o ment of my invention and practically an improvement in quality willin general be experienced, on account of the reduced tendency tooverwork amplifiers.

A further object of my invention is to utilize the phase difierencesbetween the several sections of the diaphragm to produce a desirabledistribution of sound in the auditorium or room where the loudspeaker isused. For this purpose the overall dimensions of the radiating surfacemust 10o.

-- be of the order of a wave length of the lowest frequency sound whichis to be closely directed. In practice it is not feasible to giveaccurate direction control to the lowest frequency components of soundradiated, since this would en- 0 tail very large structures, but veryuseful directivity effects may be obtained if components of 250 cyclefrequency and above are well directed. This would call for an area ofthe order of four feet square. It {nay prove desirable, where the noexpense of a larger structure is warranted, to car= ry the directioncontrol to considerably lower frequencies, while in other cases, smallerstructures may be employed, and still much benefit secured from propercontrol of the directive properties of the speaker.

Considering then only the sounds of high enough frequency to becontrolled, the directions they will take may be predicted by estimatingthe shape of the wave front of a sound wave leaving the speaker, and byutilizing the principle that sound is propagated normal to the wavefront. The phase difference between. the vibrations of successivesections of the diaphragm can be calculated from the well knowncharacteristics of low pass filter circuits. It is simpler however tomake the calculation in terms of thetime difference, which is seconds,except near and above the cut off frequency, L being the inductance ofone coil and C the capacity of one section. Thus if the crest of avoltage wave reaches section it at a given time, it will reach sectionn+1 later by JITG seconds. During this time sound can travel VJFCcentimeters if V is the velocity of sound in air or approximately 33000cms. per sec. The shape of the wave front may then be determinedgraphically by laying out a series of arcs, each having its center atthe middle of one of the diaphragm sections and having radii whichdiffer successively in length by This is similar to the well knownHuygens construction used wpecially in optical problems. My inventionwill be better understood from the following description when consideredin connection with the accompanying drawing, and its scope will bepointed out in the app nded claims. Referring to the drawing, Fig. 1illustrates an embodiment of my invention wherein the inductance unitsare interposed between the dif-- ferent sections of the .vibratablemember of the speaker, this member being interposedbetween a pair ofstationary members maintained at potentials different from that of thevibratable member.

Fig. 2 illustrates a similar embodiment wherein only one stationarymember is employed, mounted on one side of the vibratable member.

Fig. 3 illustrates an arrangement wherein the inductance elements areconnected between the difierent sections of stationary members locatedon opposite sides of the vibratable member, and Fig. 4 illustrates thesequence of connections designed to secure a certain directivecharacteristic.

The apparatus illustrated by Fig. 1 includes a plurality of vibratablemembers or diaphragm sections 10 which are electrically conducting andare connected to the output circuit of the-amplifier 11 through aplurality of inductance devices or reactors 13 connected between thedifferentsections 10. On one side of the vibratable members 10 aremounted a plurality of stationary members 14 which are shown asperforated metal plates, interconnected through conductors 15, andconnected to one of the amplifier circuits which for convenience I shalldesignate as the grounded terminal, through a source of steady potentialshown as a battery 16. It is obvious that a single continuous perforatedplate might equally well be employed so far as the principle ofoperation is concerned On the other side of the vibratable members 10are mounted a plurality of similar stationary members l7 interconnectedthrough connections 18 and connected to the grounded side of theamplifier circuit through a source of potential 19. It will be observedthat the potential applied to the members 14 by the source 16 ispositive and-that the potential applied to the member 1'? by the source19 is negative. Under these conditions an electrostatic stress isproduced between the vibratable members 10 and the stationary members 14and 17, and the resulting forces acting upon the vibratable members ordiaphragm sections 10 are substantially equal and opposite. If now avoltage is produced in the output circuit of the amplifier, thepotential of the members 10 is altered, and this reduces theelectrostatic force on one side and increases it on the other side,causing a movement of the diaphragm or vibratable member. This is inaccordance with the principle of operation of the well known bilateralor push-pull electrostatic sound producer. At 12 is shown the resistancealready referred to for preventing reflection of electric waves. Thisresistance should have, a value approximately equal to v IL/C.

Any means such as series resistances which produces high electricallosses in the inductances or capacities at and near the end 12 of thecircuit, will produce substantially the same effect as the resistance12.

The arrangement illustrated by Fig. 2 is similar to that illustrated byFig. 1 except that it is of the single acting or unilateral type,employing stationary electrodes on one side only of the diaphragm. Theamplifier output circuit also differs from that shown in Fig. l, in thatit employs a reactor 20 instead of a'transformer, this being a commonform of amplifier output affording an especially simple arrangement whenemployed in connection with a unilateral electrostatic speaker.

In the arrangement illustrated by Fig. 3 the vibratable members ordiaphragm sections are connected to a source of steady potential whichis indicated here as the same as that which supplies power to the platecircuit of the amplifier. The diaphragm may be one'continuous conductingsheet or separate sections with interconnect-- ing conductors 21. Thestationary members 14 and 17 are interconnected through inductanceelements 22 and 23, and are connected to the output circuit of theamplifier through a transformer 24 in such a manner that the audiofrequency potentials applied to the members 14 are opposite in phase tothe potentials applied to the members 1'7. This arrangement gives thesame result as that shown in Fig. 1, but has some advantage on the.score of simplicity. It is important that the inductances and capacitieson the two sides be closely equal, since otherwise the rate ofpropagation .of electrical impulses will be unequal on the two sides ofthe diaphragm and the desired phase opposition may be lost toward the.end of the chain. As a further insurance of maintenance of phaseopposition it may be desirable in a st-ructure such as shown in Fig. 3to connect under more auto-transformers 25 across between the elements14 and 17 at intervals, the mid-points of the windings being connectedto ance 12. 1 should have the same capacity as the single secipeasvvconductor 27 which is at stationary potential. The final sections of thestationary members 1417 are connected through damping resistors 26 tothe return lead 27 which is connected both to the cathode of theamplifier l1 and to a midterminal on the secondary circuit of thetransformer 24. This arrangement gives the sameresuits as Fig. 1, butrequires no extra sources of high potential and is safer in that theexposed outside members are at ground potential except for themodulating voltage.

The method of utilizing the phase diiierence between successivediaphragm sections, to secure a desired directive characteristic, isillustrated in connection with Fig. 4. For auditoriums it is of tendesirable to provide for considerable spreading of the sound to theright and left of the source in order to reach all of the listeners, butto permit a comparatively small degree of spreading in the verticalplane. In other words, it is desired to produce a beam of sound whichspreads horizontally like a fan. This is especially the case in singlefioor auditoriums. The prevention of avoidable radiation toward: theceiling is helpful in reducing echoes and reverberation. In order tocause the sound to spread horizontally while being confined withinnarrow limits vertically, the wave front as the sound leaves the speakershould have a cylindrical form with vertical axis. If the diaphragmsections consist of vertical strips a wave length or more high, the wavefront axis will be parallel to these strips. To give it the desiredcylindrical form, the radiation from the center strip must be ahead inphase compared with that from the strips to either side. This is broughtabout by using the sequence shown in Fig. 4. In this drawing thestationary electrodes are not shown, and it is to be understood thatthey would be connected as shown in Fig. 1. An electrical wave from theamplifier acts first on the section marked a, then on the adjacentsection b on the right, then on section c on the left, then on d-whichis next outside on the right, and finally on e on the extreme left. Thiswill not give a perfectly symmetrical wave, but probably a close enoughapproximation. If a more nearly perfect symmetry is desired, it can beobtained by arranging the strips in pairs, each pair being connectedtogether and replacing a single section in the circuit. Thus the circuitsequence would be from amplifier through coil to section a, through thesecond coil to sections b and 0, through a third coil and to sections (1and 0, through a fourth coil and to ground through the damping resist-In such an arran ement the pairs tions which they replace in thecircuit. This would in general involve dividing the total area into alarger total number of sections.

It is obvious that the principle herein outlined may be employed toproduce a wave front of other shape than that used .for illustration. Itis also obvious that this method of controlling wave shape ordirectivity is alternative to or may be supplementary to the methodwhich depends upon disposing the radiating elements over a curvedinstead of a plane surface. If all parts of the diaphragm moved inphase, as in the case of the ordinary electrostatic loud speaker, forexample, directivity can be controlled by shaping the entire radiatingarea to the same shape as the desired wave front. Thus the radiatingarea might be given a cylindrical form. An illustration of supplementaryaction of the two methods would be the case where in addition to thehorizontal divergence, a small amount of vertical which must be of verylight material.

gence in the two directions can be independently controlled.

As is well known in the art of designing electrical filters orartificial lines, there are two terminal arrangements either one ofwhich may be used. If the line consists of a number of seriesinductance-s L henrys alternated with shunt capacities C farads thetermination of the structure at both ends may be either capacities of(3/2 farads or inductances of L/Z henrys. The former gives what iscalled a 1.- line and the latter a T line. Thus for example a 11' linemight consist of five capacities C and two capacities (3/2 at the endsand with six coils of L henry between, while a corresponding T linewould have six capacities of C farads, five coils between of L henryseach, and two terminating coils of L/Z henrys each.

If the device produces a convex wave front, the sound will seem toemanate from a small source behind the screen. The sound may be cautedto seem to emanate from a small source in front of the screen byproducing a concave wave front. This can be brought about by reversingtheconnectlons from those shown in Fig. i so that section 0 is connectedto the amplifier and section a which is at the center is connected tothe damping resistance 12, in each through a coil of L/Z henrys asbefore.

An incidental advantage of my invention over the usual type ofelectrostatic loud speaker is that the reduced capacity per sectionreduces the destructiveness of a spark should a spark occur betweenelectrodes. Wlth a large capacity charged to high potential, a spark mayreadily'be hot enough to burn a hole through the diaphragm With thesmall sections a much less intense spark occurs for the same voltage.The adjacent sections may discharge through the are but to do so mustbuild up currents through the intervening coils, and this slows down thedischarge and permits the energy to be largely dissipated in the coilsinstead of in the spark.

While as I have already pointed out the subdivision and entroduction ofinductances may be applied either to the vibratable element ordiaphragm, or to the stationary electrodes, and in the latter case thediaphragm may be a continuous conducting sheet; it is practicallynecessary for best results that the parts of the dia-- phragm oppositethe several sections of stationary electrode, shall be capable ofsubstantially independent vibration. This is for the reason that theforces due to the electrostatic field differ in phase between theseveral sections and each part of the diaphragm should be capable ofvibrating in phase with the force applied to it. This mechanicalindependence of the diaphragm panels may be secured by providingmechanical supports for the diaphragm at the edges of the panels. Thus,if the electrical subdivision is of the stationary electrodes, thediaphragm, while i not necessarily electrically subdivided, ispreferably 1. The combination of a vibratable member, a stationarymember mounted in juxtaposition to said vibratable member, at least oneof said members being divided into sections in series with one another,means for producing an electrostatic field between said members, andmeans including inductance elements connected between said sections forneutralizing the capacitance between said members.

2. A sound producer comprising a vibratable means arranged in sectionsinterconnected through inductance elements, and stationary means mountedin juxtaposition to said vibratable means, said inductance elementsneutralizing the capacitance between said vibratable means and saidstationary means.

3. A sound producer comprising a diaphragm divided into sectionsconnected in series, stationary means arranged in juxtaposition to saiddiaphragm, and means including inductance elements connected betweensaid sections for neutralizing the capacitance between said diaphragmand said member.

4. A sound producer including a diaphragm divided into sections,stationary means mounted in juxtaposition to said diaphragm and dividedintosections, and means connected between the sections of said diaphragmfor neutralizing the capacitance between said diaphragm and saidstationary means.

5. A sound producer including stationary means, a vibratable memberdivided into sections connected in series and mounted in juxtapositionto said stationary means, and means con- 1 nectings'aid sectionsarranged to cause the vibrations of successive sections to differ inphase, said sections being connected in such mquence that the resultingsound wave is given a predetermined degree of curvature.

6. The combination of a vibratable member divided into sections, astationary member divided into sections interconnected with one anotherin series and mounted in juxtaposition to the sections of saidvibratable member, means to produce an electrostatic field between thesections of said stationary member and the sections of said vibratablemember, inductance means comprising elements connected between thesections of said vibratable member for neutralizing the capacitancebetween said vibratable and stationary members, and means connected incircuit with said vibratable member for preventing the reflection ofelectrical waves propagated from section to section of said vibratablemember.

7; An electrostatic loud speaker including vibratable and stationarymembers mounted in bratable and stationary members mounted injuxtaposition to one another, one of said members being divided intosections, means connected between-said sections for neutralizing thecapacitance between said members, and means for preventing reflection ofelectrical waves propagated between said sections.

9. An electrostatic loud speaker including vibratable and stationarymembers mounted in juxtaposition to one another, one of said membersbeing divided into sections, means for neutralizing the capacitancebetween said members and for causing a difierence in phase of the soundwaves emitted by different portions of said vibratable member, and meansincluding said firstnamed means to connect said sections in suchsequence as to produce a predetermined curvature of sound wave front.

10. A sound producer comprising a vibratable means arranged in sectionsinterconnected in series through inductance elements, and stationarymeans mounted in juxtaposition to said vibratable means.

11. A sound producer including a diaphragm divided into sectionsconnected in series, stationary means mounted in juxtaposition to saiddiaphragm and divided into sections, and means connected between thesections of said diaphragm for neutralizing the capacitance between saiddiaphragm and said stationary means.

12. An electrostatic loud speaker including vibratable and stationarymembers mounted in juxtaposition to one another, one of said membersbeing divided into sections connected in series, means connected betweensaid sections for neutralizing the capacitance between said members andresistance means connected in circuit with one of said members forpreventing reflection of electrical waves propagated between saidsections.

13. An electrostatic sound translating device comprising a plurality ofmembers forming plates of condensers, certain of said members beingcapable of radiating sound energy, and impedance means connected betweencertain of said members for neutralizing the capacity of saidcondensers, said condensers and said means being proportioned andcorrelated to define an electro acoustic network adapted to translatesound energy with substantially negligible attenuation over a wide rangeof audio-frequencies.

EDWARD W. IELLOGG.

